WHO sticks with current strains for next flu vaccine

Feb 17, 2011 (CIDRAP News) – Signaling that the current flu strains are likely to persist over the next several months, the World Health Organization (WHO) today recommended sticking with the current trio of vaccine strains for the Northern Hemisphere's next influenza season.

The WHO's vaccine strain advisory committee met on Feb 15 and 16 and released its recommendation on the WHO's Web site. The group recommends the following for next season's vaccine:

For the H1N1 component, a strain similar to A/California/7/2009

For the H3N2 component, a strain similar to A/Perth/16/2009

For the B component, a strain similar to B/Brisbane/60/2008-like virus

Each February, WHO experts assess the flu strains circulating throughout the globe before recommending the strains for the Northern Hemisphere's next seasonal flu vaccine. It takes about 6 months for vaccine manufacturers to grow the viruses in chicken eggs and formulate them into trivalent vaccines.

Today's recommendation also matches the one the WHO's vaccine advisers made on Sep 29, 2010, for the Southern Hemisphere. That region's flu season generally runs from May through October.

In the group's full report that accompanied the recommendation, it said the 2009 H1N1 virus is still being implicated in outbreaks and is the dominant strain across Europe and in some Asian countries. Since September the H3N2 strain has been predominant in the Americas, but in January circulation of the 2009 H1N1 gained momentum in the United States and Canada.

The 2009 H1N1 virus recently edged out H3N2 as the dominant strain in the United States, according to the most recent surveillance report from the US Centers for Disease Control and Prevention (CDC). In many parts of the world it is circulating alongside influenza B, which has mostly been the Victoria lineage, according to national and global flu surveillance reports. Detections of the former seasonal H1N1virus have been very rare, with only five countries reporting cases.

Influenza B cocirculated with the influenza A viruses in many Northern Hemisphere countries, and earlier this season was the dominant strain in some, such as Norway, Russia, and Ukraine, the WHO said.

Flu during the Southern Hemisphere's summer months has been generally low, with some countries reporting 2009 H1N1 circulation, with H3N2 transmission occurring in South America and other locations.

Sporadic H5N1 avian influenza cases have been reported from Cambodia, Hong Kong, Egypt, and Indonesia, and no H9N2 cases have been reported during the September-to-January period. Since September, eight zoonotic infections causes by swine H1N1 and swine H3N2 were detected across the globe, six of which were in the United States, with Switzerland and China each reporting one case.

Hemagglutination inhibition testing of circulating 2009 H1N1 viruses suggests they remain closely related to the vaccine virus A/California/7/2009. Genetic analysis of the hemagglutinin gene found that though the 2009 H1N1 viruses fell into three genetic subgroups, they were antigenically indistinguishable.

Similar studies on the H3N2 viruses found phylogenic subgroups within two clades, but all viruses were antigenically similar to Perth/16/2009. The same analyses of influenza B viruses found that both Victoria and Yamagata lineages are circulating, but Victoria continues to predominate globally, except in China, where most influenza B is antigenically closely related to the Brisbane/60/2008.

In other flu vaccine news, WHO advisors also recommended the development of two new candidate H5N1 avian flu vaccine viruses. Both are clade 2.3.2, which have been detected in wild birds in Hong Kong, Japan, and South Korea and have infected poultry in Japan, Nepal, South Korea, and Vietnam. The clade was implicated in a human H5N1 infection that occurred last year in Hong Kong. Antigenic testing found that clade 2.3.2 viruses are heterogeneous, and not all react well to postinfection ferret antiserum raised to the candidate vaccine virus A/common magpie/Hong Kong/5052/2007.

The WHO's new vaccine candidate recommendations include an A/Hubei/1/2010-like virus, the one linked to the human infection, and an A/barn swallow/Hong Kong/D10-1161/2010-like virus.

The group didn't recommend a new H9N2 candidate vaccine virus, noting that work is continuing on the one proposed in February 2010 during its recommendations for the Northern Hemisphere's 2010-11 flu season. Each year the WHO analyzes the H5N1 and H9N2 viruses to determine if any vaccine viruses changes are needed as part of its pandemic preparedness activities.